Separation of a substance mixture consisting of hydrogen chloride and phosgene

ABSTRACT

The invention relates to a process for the partial or complete separation of a mixture comprising hydrogen chloride and phosgene, possibly solvents and possibly low boilers and inerts as are typically obtained in the preparation of isocyanates by reaction of amines with phosgene, which comprises firstly carrying out a partial or complete condensation of phosgene, then a distillation or stripping step in a column to remove the hydrogen chloride from the bottom product phosgene and subsequently a scrub of the top product hydrogen chloride by means of the process solvent to absorb the phosgene in the process solvent. An after-purification by means of adsorption on activated carbon or by another suitable method can subsequently be carried out to remove solvent residues.

The invention relates to a process for the partial or completeseparation of a gaseous or completely or partly condensed mixture ofhydrogen chloride (HCl), phosgene (COCl₂) and possibly inert gasesand/or low boilers such as nitrogen, by-products from the phosgenesynthesis, e.g. carbon monoxide, chlorine, methane, carbon tetrachloride(CCl₄) and chloroform (CHCl₃) and other low-boiling components and alsosolvent which is intentionally or unintentionally present, e.g.chlorinated, preferably aromatic hydrocarbons such as dichlorobenzene,chlorobenzene, trichlorobenzene, aromatic or aliphatic hydrocarbons suchas toluene, xylene, benzene, pentane, hexane, heptane, octane,cyclohexane, biphenyl, ketones such as 2-butanone, methyl isobutylketone, esters such as diethyl isophthalate, ethyl acetate, butylacetate, nitriles such as acetonitrile or sulfolane, as is typicallyobtained in the reaction of aliphatic or aromatic amines with phosgeneto form the corresponding isocyanates.

Similar mixtures of phosgene and hydrogen chloride are also obtained,for example, in the preparation of chlorinated organic compounds such asacid chlorides or chloroformates.

Aromatic isocyanates such as TDI (tolylene diisocyanate) and MDI(methylenedi(phenyl isocyanate)) or PMDI (polymethylene-polyphenylenepolyisocyanate) and aliphatic isocyanates, such as HDI(hexamethylenedi(phenyl isocyanate)) and IPDI (isophorone diisocyanate)are prepared industrially around the world by reaction of thecorresponding amines with phosgene. In these syntheses, hydrogenchloride is liberated as by-product in the process, usually in gaseousform. In addition, a more or less large excess of phosgene is generallyused in the reaction, so that the phosgene used does not reactcompletely with the amine despite high chemical yields in the productionof isocyanates. At least part of the excess phosgene generally turns upin gaseous form together with the hydrogen chloride liberated and partor all of it is separated off from the reaction mixture together withthe hydrogen chloride. Depending on the method of separation, more orless large amounts of solvent can also be separated off or entrainedwith the mixture of hydrogen chloride and phosgene. It is also possiblefor inert gases and low boilers such as nitrogen which may have beencarried over from the phosgene synthesis or have been deliberately addedor introduced as a result of the nature of the process to be present inthe mixture of hydrogen chloride and phosgene. These can, however, alsobe separated off from the mixture of hydrogen chloride and phosgenebeforehand or afterwards. To operate an isocyanate synthesiseconomically, it is absolutely necessary to separate the mixture ofhydrogen chloride and phosgene, recirculate the excess phosgene to theisocyanate synthesis or to another use and pass the hydrogen chloridewhich has been separated off to a further use or dispose of it. Thisfurther use of the hydrogen chloride can be, for example, anoxychlorination for preparing vinyl chloride, a Deacon process for therecovery of chlorine which can be reused for the synthesis of phosgeneor the production of aqueous hydrochloric acid. The hydrogen chlorideobtained in the isocyanate synthesis has to meet specific purityrequirements, especially in the case of use for oxychlorination or forthe Deacon process, so that the separation of phosgene and hydrogenchloride has to be carried out using process variants in which it isensured that the hydrogen chloride after separation and work-up containsonly small amounts of by-products, in particular chlorinated solventssuch as dichlorobenzene or chlorobenzene. These chlorinated aromaticcompounds generally deactivate the catalyst in oxychlorinations.Furthermore, they, like phosgene, interfere in the production of aqueoushydrochloric acid for the food industry or other applications. Inaddition, residual amounts of phosgene in the hydrogen chloride pose anot inconsiderable hazard potential and health risk.

The continuous preparation of organic isocyanates by reaction of primaryorganic amines with phosgene has been described many times and iscarried out on an industrial scale. It is described, for example, in theKunststoffhandbuch, Volume 7 (Polyurethane), 3rd revised edition, CarlHanser Verlag, Munich-Vienna, p. 76 ff (1993). In the customaryembodiment of the process, the mixture of phosgene and hydrogen chlorideformed is usually separated by partial or complete condensation of thephosgene, by distillation or by absorption of phosgene in an inertsolvent.

In general, phosgene is firstly partly condensed out from the mixture ofphosgene and hydrogen chloride. The separation is therefore usuallycarried out either under a high pressure, preferably 10-50 bar, or atvery low temperatures, preferably about −30° C. At high pressures,phosgene can be condensed out easily and inexpensively, for example byuse of cooling water. This is economically advantageous, but the highpressure poses a great safety risk in the case of a leakage, as has beendescribed in DE 3212510. Low temperatures for separating phosgene andhydrogen chloride are generally associated with low pressures, butconsiderable energy has to be expended to produce the low temperatures.Furthermore, in accordance with Henry's law, the solubility of hydrogenchloride in phosgene is very high, so that recirculated phosgenecontains relatively large amounts of hydrogen chloride. This can lead toformation of amine hydrochloride in the phosgenation of the amine. Thephosgenation of amine hydrochlorides requires longer residence times andhigher excesses of phosgene and consequently implies a higher phosgeneholdup.

One possible way of separating the mixtures described is distillation.U.S. Pat. No. 3,544,611 describes a process for preparing organicisocyanates at a pressure in the range from 10 to 50 bar. It wassurprisingly found that carrying out the reaction at relatively highpressures, at least 10 atm gauge, leads to increased yields ofisocyanate. Furthermore, higher pressures aid the separation of phosgeneand hydrogen chloride. The first reaction step for the preparation ofisocyanates, viz. the reaction between amine and phosgene to form theintermediate carbamoyl chloride, is carried out in a mixing circuit. Thesecond reaction step, viz. the decomposition of the carbamoyl chlorideto give the isocyanate, is carried out in a reaction column installeddownstream of the mixing circuit, giving a mixture of phosgene andhydrogen chloride at the top of the column. Phosgene is condensed fromthis mixture in two stages, with the first condenser for partialcondensation being operated by means of cooling water and the secondcondenser for virtually complete condensation being operated using brineat a very low temperature. The condensed phosgene is returned to the topof the column. The phosgene is then taken off again at a liquid offtakein the enrichment section of the column and is returned to the reactionin the mixing circuit.

The separation of the residual phosgene from the reaction mixture whichis taken off at the bottom of the reaction column is carried out in afurther column. In the latter, phosgene is taken off at the top,condensed in two stages using cooling water and brine in a manneranalogous to the first column and returned to the mixing circuit for thereaction. Since the reaction is completed in the reaction column, thisphosgene taken off at the top of the second column contains only thehydrogen chloride which was dissolved in the reaction mixture at theinlet to the residual phosgene separation column.

A disadvantage of this process is that only part of the phosgene can becondensed out of the hydrogen chloride to minimize the phosgene loss.Complete separation by condensation without rectification is notpossible, so that relatively large amounts of phosgene are lost and,furthermore, the hydrogen chloride produced is strongly contaminatedwith phosgene. Both the phosgene taken off at the side offtake of thereaction column and that taken off at the top of the residual phosgeneseparation column contain hydrogen chloride. The concentration is ineach case determined according to Henry's law from the pressure,temperature and composition of the gaseous and liquid phases. A furtherdisadvantage is the high pressures which pose a high hazard potential inthe case of a leakage.

U.S. Pat. No. 3,544,611 describes an alternative separation of phosgeneand hydrogen chloride at pressures of from 15 bar to 50 bar or higherwith condensation and discharge of liquid hydrogen chloride. The vaporfrom the isocyanate synthesis reactor is fed into a small side column atwhose upper end liquid hydrogen chloride is taken off. In a manneranalogous to the above-described process variant, the reaction to formthe isocyanate is carried out in a mixing circuit and a downstreamreaction column. Likewise in an analogous manner, the major part of thephosgene is condensed in a dephlegmator at the top of the reactioncolumn and is returned to the column. The mixture of phosgene andhydrogen chloride taken off from the dephlegmator is fed into theabove-described side column which is operated at the same pressure asthe reaction column. Hydrogen chloride is condensed in the condenser atthe top of this side column and part of it is returned to this column asrunback and the other part is taken off in liquid form for further use.The column has a pure enrichment section but no stripping section andbottom circulation vaporizer and thus serves only to purify the hydrogenchloride. The purification of the phosgene taken off at the bottom ofthis column is carried out by recirculation to the top of the reactioncolumn. Liquid phosgene is taken off at a liquid offtake on the reactioncolumn in a manner analogous to the above-described first processvariant and is passed to the amine-phosgene reaction in the mixingcircuit. Likewise in a manner analogous to the first process variantdescribed, the residual phosgene is taken off at the top of a residualphosgene separation column following the reaction column, condensed intwo stages and recirculated to the mixing circuit for reaction in theamine-phosgene reaction.

A disadvantage of this process is that the second column for separatingoff and purifying the hydrogen chloride in the side column is operatedat the same pressure level as the reaction column in which the reactionto give the isocyanate takes place. This results in undesirable couplingof reaction section and work-up section. It can be more advantageous tocarry out the separation of phosgene and hydrogen chloride at higher orlower pressures than the reaction. Thus, high pressures in U.S. Pat. No.3,544,611, which are advantageous for the separation of phosgene andhydrogen chloride, also mean high temperatures in the reaction sectionof the isocyanate synthesis, which lead to secondary reactions andconsiderably reduce the yield of isocyanate. Furthermore, the reactioncolumn is used for purifying the phosgene by removing residual hydrogenchloride in the enrichment section of the reaction column. This islikewise an undesirable coupling. Furthermore, the phosgene which istaken off at the liquid offtake of the reaction column and is intendedfor the amine-phosgene reaction becomes saturated with hydrogenchloride. In accordance with the gas/liquid equilibrium, the hydrogenchloride concentration corresponding to the pressure and temperature atthis point is established in the liquid phase, viz. the phosgene. Sincethe pressure and the temperature in the reaction column are determinedby the reaction, it is not possible to set more favorable conditionsunder which, for example, the concentration of the hydrogen chloride inthe phosgene is significantly lower independently of the reaction.

A particular disadvantage of the second process variant is thecondensation of the hydrogen chloride which, despite the high pressures,is carried out at very low temperatures with a high energy consumption.In the case of leakage, the high pressures pose a high hazard potential.The fact that hydrogen chloride is obtained as liquid is alsoenergetically disadvantageous. In general, the hydrogen chlorideobtained in isocyanate production is processed further in gaseous form,for example in an oxychlorination to produce ethylene dichloride, sothat the hydrogen chloride has to be vaporized again with furtherconsumption of energy.

GB 827376 describes a continuous process for preparing aromaticisocyanates by reaction of an amine in free form or as salt which canreadily be decomposed to the free amine with a solution of phosgene at apressure of greater than 3*10⁵ Pa, in which the reactants are introducedsimultaneously with mixing into the lower end of a vertical tube reactorin which the reaction products rise rapidly to the upper end. The liquidphase is collected in a container from which it is taken off forisolation of the isocyanate. This container can be a phase separationapparatus which is operated under the same pressure, is connected via anoverflow tube to the liquid outlet and has a throttle valve in theliquid outlet. The liquid separated off in the container is fed to acolumn which is operated under atmospheric or superatmospheric pressureand elevated temperature and in which residual phosgene and hydrogenchloride are separated off at the top in gaseous form. The excessphosgene is condensed from the mixture of phosgene and hydrogen chlorideseparated off in the container, and the hydrogen chloride which has beenseparated off in this way is depressurized and discharged. The reactantsare fed into the tube reactor by means of a joint pump or two separatepumps or are mixed in a Venturi mixing nozzle and from there introducedinto the tube reactor. The temperature in the tube reactor is said to be80-200° C. and the pressure is greater than 3*10⁵ Pa, at most the vaporpressure of the reaction mixture and preferably from 15 to 20*10⁵ Pa.

A disadvantage is the very low purity of the hydrogen chloride, sinceconsiderable amounts of phosgene remain in the hydrogen chloride in asingle-stage condensation at the preferred pressures of from 15 to20*10⁵ Pa and these are very troublesome in further use of the hydrogenchloride for oxychlorination, for a Deacon process or for producingaqueous hydrochloric acid. A further disadvantage is the high hydrogenchloride content of the phosgene condensed out, which is recycled to thereaction without further purification.

U.S. Pat. No. 3,381,025 describes a process for preparingmethylenebis(phenyl 4-isocyanate) (MDI), tolylene 2,4-diisocyanate(TDI), polymethylene-poly(phenyl isocyanate) (polymeric MDI). Use ismade here of the principle of evaporative cooling, i.e. a mixture ofsolvent, phosgene and hydrogen chloride vaporizers at the reactiontemperature. An advantage of this is that the partial pressure of thephosgene is reduced by the presence of the solvent and phosgene cantherefore be condensed more easily. The condensation of the phosgenetogether with the chlorobenzene used as solvent is achieved by coolingthe gas mixture. The phosgene/solvent mixture is recirculated to thereaction, and the hydrogen chloride together with residual amounts ofphosgene is removed in an absorber. A 22% strength by weight mixture ofphosgene in chlorobenzene is obtained. A disadvantage of this process isthe condensation of the phosgene/solvent mixture by cooling to the verylow temperature of −30° C. Furthermore, the high loss of phosgene in thedischarged hydrogen chloride of 3.4 mol % of phosgene in the hydrogenchloride is economically disadvantageous and poses safety problems. Formost applications such as oxychlorination, a Deacon process or thepreparation of hydrochloric acid, it is not possible to use thishydrogen chloride stream. A further disadvantage is the high hydrogenchloride content of the phosgene/solvent mixture obtained, since thesolubility of hydrogen chloride in this mixture at low temperatures isparticularly high.

In U.S. Pat. No. 3,381,025, the phosgenation of an organic primary amineto form the corresponding isocyanate is carried out in two stages at atemperature of <60° C. in the first stage and from 100 to 190° C. in thesecond stage. A mixture of the inert solvent, excess phosgene and thehydrogen chloride formed is taken off from the second stage and thehydrogen chloride is separated off from this by cooling the mixture to−20° C. The cold mixture of phosgene and solvent obtained isrecirculated to the first reaction stage. A disadvantage here is theintensive cooling of the mixture which is necessary, so that high energycosts and capital costs for refrigeration equipment are incurred. Thiscooling is particularly disadvantageous when the mixture has to bereheated to the reaction temperature. A further disadvantage is the highhydrogen chloride content of the phosgene/solvent mixture obtained.

WO 99/11597 describes the separation of hydrogen chloride and phosgeneunder superatmospheric pressure in a column which is installeddownstream of a reactor for the preparation of chloroformate. Thereactor is operated at pressures of from 2 to 60 bar, preferably from 6to 40 bar. High pressures are recognized as advantageous for theseparation of phosgene and hydrogen chloride, since the condensers thendo not have to be operated at low temperatures.

A further possible way of separating phosgene and hydrogen chloride isscrubbing. SU 1811161 describes a process for preparing isocyanates byreaction of amines with phosgene. For the reaction with the amine, asolution of phosgene in chlorobenzene as solvent is prepared byabsorption of gaseous phosgene in chlorobenzene. The advantage is thesaving in energy costs, since only a one-off condensation and no furthervaporization of the phosgene is required. Amine and phosgene solutionare reacted in a reactor. In a phase separator, gas phase and liquidphase comprising isocyanate, chlorobenzene and phosgene are separated.The gas phase taken from the phase separator comprises hydrogenchloride, phosgene and chlorobenzene and is partially condensed andpassed to an absorber. The liquid phase is fed to a stripping column inwhich hydrogen chloride and phosgene are separated off at the top,partially condensed and then likewise passed to the absorber. Theincondensible gas from the stripping column is passed to the phosgenesynthesis. In the absorber, the phosgene is absorbed in chlorobenzenewhich has been distilled off from the isocyanate/chlorobenzene mixture.The phosgene/chlorobenzene stream contains about 70% by weight ofphosgene and is combined with a phosgene/chlorobenzene stream from thesynthesis of phosgene with subsequent absorption in chlorobenzene andthe combined stream is used as phosgene solution for the amine-phosgenereaction. The hydrogen chloride stream going over at the top of theabsorber is passed to further treatment, since it still contains about4% by weight of phosgene.

The document says nothing about temperatures and pressures at which theabsorber or the reactor are operated. A disadvantage of the processdescribed in SU 1811161 is the low purity both of the gaseous hydrogenchloride obtained, which contains about 4% by weight of phosgene, andalso the liquid phosgene/chlorobenzene solution which containsrelatively large amounts of hydrogen chloride because of the lowtemperatures of the chlorobenzene used for scrubbing. Owing to the highphosgene concentration, the hydrogen chloride stream cannot be used forthe customary purposes, e.g. oxychlorination, the Deacon process or theproduction of hydrochloric acid. It is generally known and alsopublished, inter alia, in I. I. Konstantinov, A. I. Kormucheshkina,Zhurnal Prikladnoi Khimii, 49 (3), pp. 596-599, 1976, and DE 3212510,U.S. Pat. No. 3,574,695 or DE 3323882 that hydrogen chloride reacts withamines in the phosgenation to form hydrochlorides whose conversion intothe isocyanate requires considerably longer residence times, highphosgene excesses and high temperatures. In addition, increasedby-product formation is observed. The saturation of the phosgenesolution with hydrogen chloride for the reaction leads to largerapparatuses, a higher phosgene holdup, a higher energy consumption and areduced yield.

EP 0570799 describes a process in which the reaction between amine andphosgene to give the isocyanate is carried out in the gas phase. Thehydrogen chloride/phosgene mixture formed is separated by means ofcondensation, absorption of phosgene in an inert solvent, for examplechlorobenzene or dichlorobenzene, maintained at from −10° C. to 8° C.,or by adsorption and hydrolysis on activated carbon.

U.S. Pat. No. 3,226,410 describes a continuous process for preparingaromatic isocyanates by mixing a stream of an aromatic amine into aphosgene stream in a tube reactor at Reynolds numbers of greater than2100, preferably 5000-2 000 000, and temperatures of from 60 to 90° C.,preferably from 80 to 85° C. The amount of phosgene is at least 1 mol,preferably from 6 to 12 mol, per mole of amine. The reaction solution isthen, if desired after pretreating, transferred to a second reactorwhich is configured as a vessel or column and is at from 110 to 135° C.,preferably from 110 to 120° C. The amine concentration is from 2 to 25%by weight, preferably from 5 to 10% by weight, and the phosgeneconcentration is from 10 to 100% by weight, preferably from 10 to 60% byweight. The pressure under which the phosgene stream is fed into thetube reactor is from 50 to 170 psig. The liquid phase comprisingisocyanate, solvent, relatively small amounts of by-products, hydrogenchloride and phosgene dissolved in the solvent is taken off from thesecond reactor separately from the gas phase comprising hydrogenchloride, solvent, phosgene and traces of isocyanate. Solvents used arechlorinated hydrocarbons which are inert and have a boiling point lowerthan that of the isocyanate. Particular preference is given tochlorobenzene. The second reactor, which is operated at a pressure of 45psig or above, is followed by a residence vessel and a buffer vesselfrom which the liquid phase is conveyed under level control to a columnfor removal of excess phosgene. Phosgene, hydrogen chloride and solventare taken off at the top and recirculated to the phosgene container. Thebottom product, comprising isocyanate and solvent, is fed to asingle-stage removal of solvent by distillation. The solvent which hasbeen separated off from the isocyanate is used for absorption of theresidual phosgene from the hydrogen chloride stream. The phosgene takenoff in the second reactor and in the buffer vessel is condensed in twostages and recirculated to the phosgene container. The uncondensedphosgene/hydrogen chloride mixture is fed to a scrubber which issupplied with solvent recovered in the solvent removal. The gas whichhas not been absorbed, mainly hydrogen chloride, is subsequently reactedwith water in an absorber to give aqueous hydrochloric acid.

Chemical separation of phosgene and hydrogen chloride is also known. WO9831662 describes the preparation of 3-(isocyanatomethyl)hexamethylene1,6-diisocyanate by phosgenation of the corresponding amine. A tertiaryamine is added as hydrogen chloride scavenger and precipitates as aminehydrochloride. The slurry is distilled in the presence of a hydrogenchloride scavenger. In DE 1233854, too, the hydrogen chloride formed isbound by addition of a tertiary amine. The addition of organic nitrogenbases is also employed in DE 3000524 and U.S. Pat. No. 3,211,776.

In JP 09208589, an alkali metal compound or alkaline earth metalcompound, preferably a weakly acidic salt or oxide such as calciumoxide, is used for scavenging the hydrogen chloride formed in thereaction.

WO 9530663 describes the synthesis of functionalized. 1,3,5-triazineisocyanates in which at least part of the hydrogen chloride is likewisebound chemically during the reaction.

A disadvantage of these processes is the solids formed, which aredifficult to handle from a chemical engineering point of view. Thework-up is made considerably more difficult since the ammonium saltsformed frequently crystallize out. Furthermore, the introduction of anadditional auxiliary makes the process uneconomical. The auxiliaries canalso result in formation of various by-products in the phosgenation,which reduce the quality of the desired isocyanate.

It is an object of the present invention to separate mixtures ofphosgene and hydrogen chloride inexpensively, i.e. with a low energyconsumption, and with a low hazard potential from a safety point of viewinto hydrogen chloride having a good purity and pure phosgene.

If dissolved hydrogen chloride remains in the recirculated phosgene inthe separation of phosgene and hydrogen chloride, this reacts directlywith the free amine in the first stage of the phosgenation to form aminehydrochloride. However, the reaction rate of the hydrochloridephosgenation is considerably lower than that of the free amine, asdescribed in I. I. Konstantinov, A. I. Kormucheshkina, ZhurnalPrikladnoi Khimii, 49 (3), pp. 596-599, 1976. In addition, the reactionof amine hydrochlorides with phosgene to form the isocyanate requireshigher temperatures and therefore tends, as described in GB 1212249, togive increased by-product formation, in particular the formation ofureas. Avoiding the formation of amine hydrochlorides thus leads tosmaller apparatuses, a lower phosgene holdup, a lower energy consumptionand higher yields. Since the solubility of amine hydrochlorides in therespective reaction mixtures and also in most commercially availablesolvents is very low, formation of hydrochloride also leads to theproblem of solids formation. It is therefore important that thesolutions of amine and phosgene which are reacted contain no dissolvedhydrogen chloride.

The hydrogen chloride formed in situ by the reaction of amine withphosgene can likewise form amine hydrochloride in a subsequent reactionwith the amine still present. However, it has been found that there is aconsiderable difference in the order of magnitude of hydrochlorideformation and also the further reaction with phosgene depending onwhether the hydrogen chloride is brought into contact with the aminewhich is present in high concentration immediately at the beginning ofthe reaction and forms amine hydrochloride, which precipitates as asolid, virtually quantitatively, or whether hydrogen chloride formedonly in the course of the reaction reacts with the amine which is thenonly present in a very low concentration to form amine hydrochloride.The reaction between amine and phosgene is extremely fast even at lowtemperatures, so that the amine concentration drops sharply at thebeginning of the reaction. Amine hydrochloride from hydrogen chlorideformed in situ displays high supersaturation in the reaction mixture,with the isocyanate which is already present in a relatively highconcentration acting as solubilizer. It therefore reacts relativelyquickly with phosgene to form carbamoyl chloride, in contrast to aminehydrochloride which has been formed directly from amine and hydrogenchloride and has precipitated as a solid.

The present invention accordingly provides a process for the partial orcomplete separation of a mixture comprising hydrogen chloride andphosgene, possibly solvents, low boilers and inerts as are typicallyobtained in the preparation of isocyanates by reaction of amines withphosgene, which comprises firstly carrying out a partial or completecondensation of phosgene, then a distillation or stripping step in acolumn to remove the hydrogen chloride from the bottom product phosgeneand subsequently a scrub of the top product hydrogen chloride by meansof the process solvent to absorb the phosgene in the process solvent. Toremove solvent residues from the phosgene and/or hydrogen chloride, theycan subsequently be purified further by means of adsorption, for exampleon activated carbon, or by other suitable methods.

In the process of the present invention, the partial or completeseparation of a mixture of hydrogen chloride and phosgene and possiblythe other components mentioned is carried out by partial condensation,in one or more stages and at various temperature and pressure levels,subsequent distillation or stripping in a column to remove hydrogenchloride from the phosgene and subsequent absorption of the phosgeneremaining in the hydrogen chloride stream in the process solvent. Thelatter is preferably chlorobenzene, dichlorobenzene, mixtures of the twoor toluene.

Largely pure hydrogen chloride is obtained at the top of the absorber,and this can be passed to a further use. The phosgene stream obtained atthe bottom of the distillation column is recirculated to the isocyanatesynthesis, preferably for reaction with the amine in the first stage,for example in a static mixer. However, it can also be fed to anotherapparatus of the reaction section or work-up section. Thephosgene/scrubbing medium stream obtained at the bottom of the absorberis likewise recirculated to the isocyanate synthesis, i.e. to the firstreactor or one of the subsequent reactors or to a column for phosgeneseparation or for work-up of the reaction mixture. In particular, thisstream can be used as runback for a column, for example a reactioncolumn. This is particularly advantageous because it comprises not onlyphosgene and the scrubbing medium but also hydrogen chloride which wouldlead to amine hydrochloride if recirculated to the first reactor. Ifdesired, the hydrogen chloride stream leaving the absorber can besubjected to an after-purification, in particular an adsorption onactivated carbon, a pressure swing adsorption, a further scrubbing stepor some other after-purification process. Undesirable components, inparticular chlorinated hydrocarbons such as the solvents dichlorobenzeneor chlorobenzene can be removed from the hydrogen chloride stream inthis way.

The mixture of phosgene and hydrogen chloride used for the process ofthe present invention usually comprises hydrogen chloride and phosgenetogether with, possibly, solvents such as dichlorobenzene,chlorobenzene, toluene or others and, possibly, low boilers and inertssuch as nitrogen, carbon monoxide, methane, carbon tetrachloride orchloroform.

The partial condensation of phosgene from the resulting mixturecomprising hydrogen chloride, phosgene, possibly solvents and inerts iscarried out in one stage or preferably more than one stage at −40° C.,achievable by means of refrigerants, to 40° C., achievable by means ofcooling water, depending on the pressure in the reaction section. Thedistillation to remove hydrogen chloride from the recirculated phosgeneis carried out at a temperature at the bottom of from 5 to 150° C.,preferably from 5 to 50° C., a pressure at the top of from 1 to 35 bar,preferably from 1.5 to 4.0 bar and a temperature at the top of from −20°C. to 30° C., preferably from −10° C. to 0° C. As an alternative, thehydrogen chloride can also be removed from the recirculated phosgene bystripping with an inert gas such as nitrogen, the process solvent vapor,phosgene or another gaseous or vaporizable substance. The temperature atthe top of the absorber is from −40° C. to 10° C., preferably from −15°C. to 0° C., the temperature at the bottom is from −10° C. to 30° C.,preferably from 0 to 10° C., and the pressure at the top is from 1 to 35bar, preferably from 1.5 to 4.0 bar. The absorption medium stream canadvantageously be saturated with hydrogen chloride beforehand, so thatsaturation of the absorption medium with hydrogen chloride does not takeplace in the scrubber. The liberated heat of solution of hydrogenchloride in the absorption medium stream can then be removed externallyin a heat exchanger. Removal of the liberated heat of the solution ofhydrogen chloride in the absorption medium stream can alternatively becarried out using intermediate cooling on the absorber.

The upstream partial condensation is advantageous from an energy pointof view, since the condensation can be carried out stepwise at varioustemperature and, if appropriate, pressure levels. The subsequentdistillation gives a phosgene stream which is largely free of hydrogenchloride at the bottom, so that considerably less amine hydrochloridecan be formed in the reaction with amine.

The advantage of the subsequent absorption of phosgene from the hydrogenchloride stream is a saving in energy costs, since condensation andrenewed vaporization of the phosgene remaining in the hydrogen chloridestream is not necessary. Particular energy savings can be achieved byfeeding the phosgene-containing output from the absorber to thephosgenation reactor as feed or, if appropriate, to a reaction column ora column for phosgene separation or for work-up of the reaction mixtureas runback. In the latter cases, it may in this way be possible to omitthe condenser at the top of the column and dispense with the partial orcomplete condensation of the vapor stream for generating runback. Thisconsiderably reduces the energy consumption.

The invention is illustrated by the following example.

EXAMPLE

The separation of a hydrogen chloride/phosgene mixture was carried outby partial condensation, distillation in a bubble cap tray column andsubsequent absorption in a downstream scrubber, by means of which verypure phosgene can be produced at the bottom outlet of the distillationcolumn and pure hydrogen chloride can be produced at the top outlet ofthe scrubber in an economically advantageous manner and at the same timeat low pressures and thus a reduced hazard potential. Solvent residuesare removed by means of adsorption from the hydrogen chloride which hasbeen separated off in this way.

1) Partial Condensation of Phosgene:

The partial condensation of phosgene from a stream from an isocyanatesynthesis was carried out in two successive heat exchangers at 33° C.(cooling water) and at −15° C. (brine). The hydrogen chloride/phosgenemixture used had a flow rate of 3.26 kg/h. The stream used comprisedphosgene, hydrogen chloride, chlorobenzene and the low boilers andinerts typical for an isocyanate synthesis. The composition was 2.43kg/h of phosgene (74.6% by weight), 0.235 kg/h of hydrogen chloride(7.2% by weight), 0.56 kg/h of chlorobenzene (17.3% by weight), 0.0106kg/h of CCl₄ (0.3% by weight), 0.0169 kg/h CHCl₃ (0.5% by weight), 0.001kg/h of carbon dioxide (0.02% by weight) and small amounts of nitrogen(4 ppm). The mixture obtained was fed as a two-phase mixture into themiddle part of the downstream distillation column.

2) Distillation to Remove Hydrogen Chloride from the Phosgene so as toRecover Very Pure Phosgene:

A bubble cap tray column having 33 bubble cap trays, 22 in the strippingsection and 11 in the enrichment section, was used. The diameter of thecolumn was 55 mm. The pressure at the top was 2.5 bar abs. and thetemperature at the bottom was 38° C. The temperature at the top of thecolumn was −9° C. A Robert vaporizer was used as vaporizer and ashell-and-tube apparatus having 13 tubes was used as condenser at thetop.

The mixture obtained from the partial condensation was fed into thecolumn between the stripping and enrichment sections. The feedtemperature was 24.5° C. The feed, viz. the hydrogenchloride/phosgene/chlorobenzene mixture, was introduced between thestripping and enrichment sections.

At the bottom, a total stream of 5.53 kg/h comprising 4.9 kg/h ofphosgene (89% by weight), 0.0005 kg/h of hydrogen chloride (100 ppm),0.57 kg/h of chlorobenzene (10% by weight), 0.02 kg/h of CHCl₃ (0.3% byweight) and 0.01 kg/h of CCl₄ (0.2% by weight) was taken off. Thephosgene/chlorobenzene mixture taken off at the bottom of the columnthus contained only small traces of hydrogen chloride, i.e. the phosgenewas very pure in respect of hydrogen chloride. The inert solventchlorobenzene does not interfere in the recirculation to the reactionsection.

The phosgene/hydrogen chloride stream taken off at the top of the columnwas partially condensed in the condenser at the top and was returned tothe column as runback. The amount of runback was 0.2 kg/h and itstemperature was −20° C. The incondensible stream of 1.24 kg/h passingthe condenser at the top had the following composition: 0.92 kg/h ofhydrogen chloride (74% by weight), 0.32 kg/h of phosgene (26% byweight), 0.2% by weight of carbon dioxide, 0.001 kg/h of carbon monoxide(800 ppm) and small amounts of nitrogen (140 ppm).

3) Absorber (Scrubber):

Removal of Phosgene by Scrubbing the Hydrogen Chloride Stream

A packed column which had a diameter of 30 mm and had three sections ofpacking containing 3×3 mm wire mesh rings was used. The bed height was630 mm per section. The pressure at the top was 2.2 bar abs. and thetemperature at the bottom was 6° C. The temperature at the top of thecolumn was −8° C. The column was equipped with a condenser at the top. Ademister was installed in the top of the column to avoid entrainment ofdroplets.

The hydrogen chloride/phosgene feed stream had a flow of 1.24 kg/h andhad the abovementioned composition. The feed temperature was −20° C. Achlorobenzene stream having a temperature of −25° C. and a flow of 1.32kg/h was introduced at the top of the scrubber. The composition was 1.3kg/h of chlorobenzene (99.6% by weight), 0.003 kg/h of CCl₄ (0.2% byweight) and 0.002 kg/h of CHCl₃ (0.2% by weight). Intermediate coolingwas used to remove the liberated heat of solution of hydrogen chloridein the chlorobenzene scrubbing medium. The intermediate cooler wasoperated at −30° C.

The vapor stream obtained at the top of the scrubber was passed to thecondenser at the top at which, in particular, entrained droplets wereprecipitated. The condensible fraction was recirculated to the bottom ofthe scrubber. The incondensible fraction of 0.87 kg/h had a compositionof 0.86 kg/h of hydrogen chloride (99.5% by weight), 0.001 kg/h ofphosgene (0.1% by weight), 0.001 kg/h of chlorobenzene (0.1% by weight),0.001 kg/h of carbon dioxide (0.1% by weight), 0.001 kg/h of carbonmonoxide (0.1% by weight) and 0.0005 kg/h of N₂ (500 ppm) and in eachcase small amounts of CCl₄ and CHCl₃ phosgene (each 80 ppm). Thecondenser at the top was operated at −30° C. The hydrogen chloridestream obtained in this way is very pure (99.9% by weight). Thechlorobenzene/phosgene stream of 1.69 kg/h taken off at the bottom ofthe column has the composition 0.32 kg/h of phosgene (19% by weight),1.3 kg/h of chlorobenzene (78% by weight), 0.05 kg/h of hydrogenchloride (3% by weight), 0.0027 kg/h of CCl₄ (0.2% by weight), 0.002kg/h of CHCl₃ (0.1% by weight) and small amounts of nitrogen (60 ppm).

4) After-Purification of Hydrogen Chloride from the HydrogenChloride/Phosgene Separation

The gaseous output from the top of the absorber was passed through anactivated carbon filter in which residual phosgene and chlorobenzenewere adsorbed on the activated carbon. A very pure hydrogen chloridestream in which no phosgene or chlorobenzene could be detected byin-line IR or GC analysis was obtained.

1. A process for the partial or complete separation of a mixturecomprising hydrogen chloride and phosgene, which comprises: firstlycarrying out a partial or complete condensation of phosgene; thendistillation or stripping in a column to remove the hydrogen chloridefrom the bottom product phosgene; and subsequently a scrub of the topproduct hydrogen chloride by a process solvent to absorb the phosgene inthe process solvent.
 2. The process as claimed in claim 1, wherein thepartial or complete condensation of phosgene is carried out at from −40°C. to 40° C. and pressures of from 1 to 35 bar.
 3. The process asclaimed in claim 1, wherein the distillation to remove hydrogen chloridefrom phosgene is carried out at a temperature of the bottom of from 5 to150° C., a pressure at the top of from 1 to 35 bar, and a temperature atthe top of from −20° C. to 30° C.
 4. The process as claimed in claim 1,wherein the hydrogen chloride is removed from the phosgene by strippingwith an inert gas.
 5. The process as claimed in claim 1, wherein theabsorption or scrub is carried out by the process solvent.
 6. Theprocess as claimed in claim 1, wherein the temperature at the top of theabsorber is from −40° C. to 10° C., the temperature at the bottom isfrom −10° C. to 30° C., and the pressure at the top is 1-35 bar.
 7. Theprocess as claimed in claim 1, wherein an absorption medium stream forthe absorption has been saturated beforehand with hydrogen chloride. 8.The process as claimed in claim 1, wherein the heat of condensation ofhydrogen chloride and phosgene in an absorption medium is removed byintermediate cooling in the absorber.
 9. The process as claimed in claim1, wherein an after-purification by adsorption is carried out.
 10. Theprocess as claimed in claim 1, wherein the scrub is carried out bychlorobenzene.
 11. The process as claimed in claim 1, wherein the bottomproduct phosgene is recirculated to the reaction section of anisocyanate synthesis.
 12. A runback in distillation or reaction columns,or a scrubbing solution for absorbers or scrubbers, comprising thebottom product phosgene as claimed in claim
 1. 13. The process asclaimed in claim 1, wherein the hydrogen chloride obtained issubsequently compressed.
 14. The process as claimed in claim 1, whereinthe hydrogen chloride obtained is subsequently added in a preparation ofethylene dichloride or a Deacon process.
 15. The process as claimed inclaim 1, wherein the mixture further comprises solvents, low boilers,and inert gases obtained in a preparation of isocyanates by reaction ofamines with phosgene.
 16. The process as claimed in claim 7, wherein aheat of condensation has been removed.